Recently, an electroluminescence (EL) device using organic materials (organic electroluminescence device), in place of inorganic materials such as ZnS, has been developed. One of the important issues with an organic electroluminescence device is how to achieve high luminous efficiency. In this respect, a great progress has been made by use of a hole-transport layer containing aromatic amines and an emitting layer containing 8-hydroxyquinoline aluminum complex.
An important problem that has to be overcome to expand the demand for an organic electroluminescence device is how to lower its driving voltage. For example, it is required that display devices of portable instruments can operate at a low driving voltage from a battery. For its general use also, apart from portable instruments, the cost of driving IC depends on driving voltage, and the cost becomes lower as the driving voltage is lowered. Gradual increase in driving voltage on continuous use also presents a serious problem in maintaining stability in performance of display devices.
In order to solve these problems, attempts are being made to mix an electron-accepting compound with a hole-transporting compound.
For example, in Patent Document 1, it is indicated that, by mixing tris(4-bromophenyl aminiumhexachloroantimonate) (TBPAH) as an electron-accepting compound with a hole-transporting macromolecule compound, it is possible to obtain an organic electroluminescence device capable of functioning at a low voltage.
In Patent Document 2, a method is described, wherein an electron-accepting compound ferric chloride (III) (FeCl3) is mixed with a hole-transporting compound by means of vacuum vapor deposition.
In Patent Document 3, a method is described, wherein an electron-accepting compound tris(pentafluorophenyl)borane (PPB) is mixed with a hole-transporting macromolecule compound by means of a wet coating method to form a hole-injection layer.
When a hole-transporting compound is mixed with an electron-accepting compound, electrons are transferred from the hole-transporting compound to the electron-accepting compound, and an ionic compound is formed which consists of a cation radical of the hole-transporting compound and a counter anion originating from the electron-accepting compound.
When TBPAH described in Patent Document 1 is used as an electron-accepting compound, the counter anion is SbCl6−. When FeCl3 described in Patent Document 2 is used as an electron-accepting compound, the counter anion is Cl− (or FeCl4−). When PPB described in Patent Document 3 is used as an electron-accepting compound, the counter anion is an anion radical shown below.

(An anion radical means a chemical species having unpaired electrons and negative charge. The negative charge is thought to be spread over the entire molecule. However, in the above chemical formula, the resonance structure thought to have the greatest possible contribution is shown.)
In Patent Document 4, an ionic compound consisting of an aminium cation radical and SbF6− or BF4− is indicated as a component of a charge-transport film of a photovoltanic instrument (organic solar battery).
Patent Document 5 suggests the use of an ionic compound consisting of an aminium cation radical and a counter anion as a component of an electro-conductive coating film (charge-transport film). As counter anions are exemplified halide ion such as I−, polyhalide ion such as Br3−, oxonic acid ion such as ClO4− and PO3−, ion consisting of center ion and halogen such as BF4−, FeCl4−, SiF62− and RuCl62−, carboxylate ion such as CF3COO−, sulfonate ion such as CF3SO2O−, ate complex originating from sulfonate ion such as (CF3SO3)4Al−, C60−, C602−and B12H122−.
Ionic compounds consisting of an aminium cation radical and a counter anion have an absorption band in the near infrared region and in Patent Document 6, an indication is given to use these compounds as infrared cut filter. Tetraphenylborate ion is exemplified as a counter anion.
[Patent Document 1] Japanese Patent Laid-Open Application No. HEI 11-283750
[Patent Document 2] Japanese Patent Laid-Open Application No. HEI 11-251067
[Patent Document 3] Japanese Patent Laid-Open Application No. 2003-31365
[Patent Document 4] Japanese Patent Laid-Open Application No. 2003-197942
[Patent Document 5] U.S. Pat. No. 5,853,906
[Patent Document 6] Japanese Patent Laid-Open Application No. 2000-229931